A halftone image may be prepared by a conventional computer, laser printer, ink printer, copier, or facsimile machine by digital computation and then may be stored in whole or in part prior to transferring the image to a tangible medium such as printable media, film, or a printing plate. A halftone image is composed of output picture elements (pixels). A halftone image is generally prepared to satisfy a limitation of the media or of the transfer process that prohibits the formation of an output pixel having an adjustable analog intensity level.
Preparation of a halftone image using the conventional clustered dot technique includes grouping output pixels to form a halftone dot that varies in size and proximity to other halftone dots to convey analog pixel intensity information (e.g., a shade of grey) that cannot be conveyed by varying output pixel intensity. Output pixels generally have a binary intensity. For example, a pixel is black when printed or white when not printed. Translation by digital computation usually introduces some quantization and possibly inaccurate variation of halftone dot size and placement. When a halftone image is transferred to tangible media, variation in halftone dot size and placement, such as introduced by a mechanical system, as well as any effects of quantization and inaccuracy may give rise to undesirable, visible patterns known generally as screening artifacts. Screening artifacts are not pleasing to the eye. For example, a large area of the same color in the original can have a pattern with objectionable variation in color in the reproduced image. Screening artifacts produce patterns not in the original object. One common screening artifact is known as a moire pattern. There are also additional screening artifacts.
In offset printing, screens for printing halftones were created on the basis of the intensity of the light from an original object. A contact screen that includes a criss-cross pattern of varying density in the emulsion was placed in contact on unexposed film. Light reflected from the original was passed through the lens of a special camera to expose the film. The light from the lens passed through the contact screen. The contact screen controlled the exposure of the film contacting the contact screen. In other words, the contact screen exposed the film in response to the amount, brightness or intensity of the light transmitted from the original. The larger the intensity, brightness or amount of light, the larger the dot size formed on the negative. A larger dot size on the negative produces a smaller the dot size when a copy of the original is produced. Also, when the amount of light, brightness or intensity is lower, the dot size produced on the film is smaller and hence a larger dot size will be produced in the original.
In electronic printing systems, a contact screen is not used. Even though a contact screen is not used, the screen for halftone printing is produced electronically based on the measured intensity or brightness or amount of light received at a picture element (pixel) from an original.